In a pneumatic control system, it is necessary to measure an output signal of a possible high order, in order to perform an accurate and rapid control. To this end, when the pressure within a vessel is to be controlled by a pneumatic servovalve, for example, an advanced derivative control (D-PI control) is often used, in which the pressure within the vessel is measured by a pressure sensor and a signal obtained by differentiating an output signal from the sensor using a differentiator is fed back as a minor loop.
Many methods in which a high-order signal is estimated by a low-order signal, as described above, have been suggested. However, it is ideal that an actual value of the high-order output signal may be directly measured by a sensor. This is because the estimation is not easy because of an affect of a sensor noise and, further, a derivative of the pressure cannot be correctly detected when a pressure change is too small and below the resolving power of the sensor.
In relation to a pressure differentiator, a method for measuring a differential pressure has been suggested. (For example, see Ernest O. Doebelin. Measurement Systems, McGran-Hill, (1976).) In other words, as a method for directly measuring a derivative of the pressure, a method for detecting a differential pressure between two sections by using a diaphragm has been suggested. This method utilizes a phenomenon in which the displacement of the diaphragm becomes a first order lag system for the pressure when pressurization of one of the sections is carried out through a capillary tube. This configuration has been used in an altimeter of an aircraft.
On the other hand, an isothermal pressure vessel has been developed. (For example, see K. Kawashima, T. Fujita, T. Kagawa: Measurement Method of Flow Rate Using Pressure Change in Vessel, Journal of the Society of Instrument and Control Engineers (SICE), Vol. 32, No. 11, 1485/1492, (1996); and K. Kawashima, T. Kagawa, T. Fujita: Instantaneous Flow Rate Measurement of Ideal Gases. Trans. ASME Journal of Dynamic Systems, Measurement and Control, Vol. 122, pp. 174-178, (2000).)